Postgres: Handle configuration parameters for db:drop and db:create

Fixes 3 minor issues:
- `rake db:drop` => give port number, host name and password to postgresql
- `rake db:create` & `db:drop` => turn port number into a string before adding it
- `rake db:create` & `db:drop` => unset password only if it was set here (It might come from the system environment rather than from config file)